Retroreflective articles including optically active areas and optically inactive areas

ABSTRACT

This disclosure generally relates to retroreflective articles and methods of making such articles.

TECHNICAL FIELD

This disclosure generally relates to retroreflective articles andmethods of making such articles.

BACKGROUND

Retroreflective materials are characterized by the ability to redirectlight incident on the material back toward the originating light source.This property has led to the widespread use of retroreflective sheetingfor a variety of traffic and personal safety uses. Retroreflectivesheeting is commonly employed in a variety of articles, for example,road signs, barricades, license plates, pavement markers and markingtape, as well as retroreflective tapes for vehicles and clothing.

Two known types of retroreflective sheeting are optical element sheeting(e.g., cube corner sheeting) and microsphere-based sheeting.Microsphere-based sheeting, sometimes referred to as “beaded” sheeting,employs a multitude of microspheres typically at least partiallyembedded in a binder layer and having associated specular or diffusereflecting materials (e.g., pigment particles, metal flakes or vaporcoats, etc.) to retroreflect incident light. Cube corner retroreflectivesheeting, sometimes referred to as “prismatic” sheeting, typicallycomprises a thin transparent layer having a substantially planar firstsurface and a second structured surface comprising a plurality ofgeometric structures, some or all of which include three reflectivefaces configured as a cube corner element.

Typically, a cube corner element includes three mutually perpendicularoptical faces that intersect at a single apex. Generally, light that isincident on a corner cube element from a light source is totallyinternally reflected from each of the three perpendicular cube corneroptical faces and is redirected back toward the light source. Presenceof, for example, dirt, water, and adhesive on the optical faces canprevent total internal reflection (TIR) and lead to a reduction in theretroreflected light intensity. As such, the air interface is typicallyprotected by a sealing film. However, sealing films may reduce the totalactive area, which is the area over which retroreflection can occur.Further, sealing films increase the manufacturing cost. Additionally,the sealing process can create a visible pattern in the retroreflectivesheeting that is undesirable for many applications, such as, forexample, use in a license plate and/or in commercial graphicsapplications where a more uniform appearance is generally preferred.Metallized cube corners do not rely on TIR for retroreflective light,but they are typically not white enough for daytime viewing of, forexample, signing applications. Furthermore, the durability of the metalcoatings may be inadequate.

SUMMARY

The inventors of the present application have formed retroreflectivearticle without sealing films and/or metallized cube corners.

Some embodiments of the retroreflective articles of the presentdisclosure include one or more optically active areas in which incidentlight is retroreflected by a structured surface including, for example,cube corner elements, and one or more optically inactive areas in whichincident light is not substantially retroreflected by the structuredsurface. The one or more optically active areas include a low refractiveindex layer or material adjacent to a portion of the structured surface.The one or more optically inactive areas include a pressure sensitiveadhesive adjacent to a portion of the structured surface. The pressuresensitive adhesive substantially destroys the retroreflectivity of theportions of the structured surface that are directly adjacent thereto.The low refractive index layer assists in maintaining theretroreflectivity of the adjacent structured surface by forming a“barrier” between the structured surface and the pressure sensitiveadhesive.

Some embodiments of the retroreflective articles of the presentdisclosure include a barrier layer between the pressure sensitiveadhesive and the low refractive index layer. The barrier layer hassufficient structural integrity to substantially prevent flow of thepressure sensitive adhesive into the low refractive index layer.Exemplary materials for inclusion in the barrier layer include resins,polymeric materials, inks, dyes, and vinyls. In some embodiments, thebarrier layer traps a low refractive index material in the lowrefractive index layer. Low refractive index materials are materialsthat have an index of refraction that is less than 1.3.

Some embodiments of the present disclosure generally relate to aretroreflective article, comprising: a retroreflective layer includingmultiple cube corner elements that collectively form a structuredsurface that is opposite a major surface; and a sealing layer having afirst region and a second region wherein the second region is raisedrelative to the first region and is in contact with the structuredsurface.

Some embodiments of the present disclosure generally relate to aretroreflective article, comprising: a retroreflective layer including astructured surface that is opposite a major surface; a sealing layerhaving a first region and a second region wherein the second region israised relative to the first region and is in contact with thestructured surface to form an optically inactive area that does notsubstantially retroreflect incident light; and the first region formingan optically active area that substantially retroreflects incidentlight.

Some embodiments of the present disclosure A method of forming aretroreflective article, comprising: providing a retroreflective layerincluding a structured surface that is opposite a second major surface;and forming a sealing layer having a first region and a second regionwherein the second region is raised relative to the first region; andattaching the sealing layer to the structured surface such that thesecond region is in contact with the structured surface and the firstregion is not in contact with the structured surface.

BRIEF DESCRIPTION OF DRAWINGS

The present disclosure may be more completely understood and appreciatedin view of the following detailed description of various embodiments inconnection with the accompanying drawings, in which:

FIGS. 1A and 1B are schematic side views of one exemplary embodiment ofa retroreflective article of the present disclosure.

FIG. 2 is schematic drawing of one exemplary intermediary step informing the retroreflective article of FIG. 1.

FIG. 3 is a schematic drawing of one exemplary embodiment of aretroreflective article of the present disclosure.

FIGS. 4A and 4B are schematic drawings of the pattern used in Examples11-13.

FIG. 5 is a schematic drawing of one exemplary embodiment of aretroreflective article of the present disclosure.

FIG. 6 is a schematic drawing of the pattern used in Examples 1-6.

FIGS. 7A and 7B are plots showing observations angles versus distancefor left and right headlights for a standard sedan.

FIG. 8 is a schematic drawing of the pattern used in Examples 14-19.

FIG. 9 is a schematic drawing of the pattern used in Examples 14-19.

DETAILED DESCRIPTION

FIGS. 1A and 1B show one exemplary embodiment of a retroreflectivearticle 100 that faces viewer 102. Retroreflective article 100 includesa retroreflective layer 110 including multiple cube corner elements 112that collectively form a structured surface 114 opposite a major surface116. Retroreflective layer 110 also includes an overlay layer 118. Apressure sensitive adhesive layer 130 is adjacent to retroreflectivelayer 110. Pressure sensitive adhesive layer 130 includes a pressuresensitive adhesive 132, one or more barrier layers 134, and a liner 136.Barrier layer 134 has sufficient structural integrity to preventpressure sensitive adhesive 132 from flowing into a low refractive indexlayer 138 that is between structured surface 114 and barrier layer 134.Barrier layer 134 can directly contact or be spaced apart from or canpush slightly into the tips of cube corner elements 112.

Where present, barrier layers 134 form a physical “barrier” betweenpressure sensitive adhesive 130 and cube corner elements 112. Barrierlayers may prevent wetting of cube tips or surfaces by the pressuresensitive either initially during fabrication of the retroreflectivearticle or over time due to the to viscoelastic nature of the adhesive.A trapped layer between pressure sensitive adhesive 130 and cube cornerelements 112 is low refractive index layer 138. Low refractive indexlayer is thereby enclosed. If a protective layer is applied thereto, thelow refractive index layer is encapsulated. Encapsulation of the lowrefractive index layer maintains and/or protects the integrity of thelow refractive index layer. The presence of the barrier layer permitsthe portions of structured surface 114 adjacent to low refractive indexlayer 138 and/or barrier layers 134 to retroreflect incident light 150.Barrier layers 134 may also prevent pressure sensitive adhesive 130 fromwetting out the cube sheeting. Pressure sensitive adhesive 130 that isnot in contact with a barrier layer 134 adheres to the cube cornerelements, thereby effectively sealing the retroreflective areas to formoptically active areas or cells. Pressure sensitive adhesive 130 alsoholds the entire retroreflective construction together, therebyeliminating the need for a separate sealing film and sealing process. Insome embodiments, the pressure sensitive adhesive is in intimate contactwith or is directly adjacent to the structured surface or the cubecorner elements.

As is shown in FIG. 1B, a light ray 150 incident on a cube cornerelement 112 that is adjacent to low refractive index layer 138 isretroreflected back to viewer 102. For this reason, an area ofretroreflective article 100 that includes low refractive index layer 138is referred to as an optically active area. In contrast, an area ofretroreflective article 100 that does not include low refractive indexlayer 138 is referred to as an optically inactive area because it doesnot substantially retroreflect incident light.

Low refractive index layer 138 includes a material that has a refractiveindex that is less than about 1.30, less than about 1.25, less thanabout 1.2, less than about 1.15, less than about 1.10, or less thanabout 1.05. Exemplary low refractive index materials include air and lowindex materials (e.g., low refractive index materials described in U.S.Patent Application No. 61/324,249, which is hereby incorporated hereinby reference).

In general, any material that prevents the pressure sensitive adhesivefrom contacting cube corner elements 112 or flowing or creeping into lowrefractive index layer 138 can be used in barrier layer 134. Exemplarymaterials for use in barrier layer 134 include resins, polymericmaterials, dyes, inks, vinyl, inorganic materials, UV-curable polymers,pigment, particle, and bead. The size and spacing of the barrier layerscan be varied. In some embodiments, the barrier layers may form apattern on the retroreflective sheeting. In some embodiments, one maywish to reduce the visibility of the pattern on the sheeting. Ingeneral, any desired pattern can be generated by combinations of thedescribed techniques, including, for example, indicia such as letters,words, alphanumerics, symbols, or even pictures. The patterns can alsobe continuous, discontinuous, monotonic, serpentine, any smoothlyvarying function, stripes, varying in the machine direction, thetransverse direction, or both; the pattern can form an image, logo, ortext, and the pattern can include patterned coatings and/orperforations. In some embodiments, the printed areas and/or unprintedareas can form a security feature. The pattern can include, for example,an irregular pattern, a regular pattern, a grid, words, graphics, imageslines, and intersecting zones that form cells.

In at least some embodiments, the pressure sensitive adhesive layerincludes a first region and a second region. The second region is indirect or intimate contact with the structured surface. The first andsecond regions have sufficiently different properties to form andseparate the low refractive index layer between and from the pressuresensitive adhesive layer and the structured surface of theretroreflective layer. In some embodiments, the second region includes apressure sensitive adhesive and the first region differs in compositionfrom the second region. In some embodiments, the first region and thesecond region have different polymer morphology. In some embodiments,the first region and the second region have different flow properties.In some embodiments, the first region and the second region havedifferent viscoelastic properties. In some embodiments, the first regionand the second region have different adhesive properties. In someembodiments, the retroreflective article includes a plurality of secondregions that form a pattern. In some embodiments, the pattern is one ofan irregular pattern, a regular pattern, a grid, words, graphics, andlines.

Exemplary pressure sensitive adhesives for use in the retroreflectivearticles of the present disclosure include crosslinked tackified acrylicpressure-sensitive adhesives. Other pressure sensitive adhesives such asblends of natural or synthetic rubber and resin, silicone or otherpolymer systems, with or without additives can be used. The PSTC(pressure sensitive tape council) definition of a pressure sensitiveadhesive is an adhesive that is permanently tacky at room temperaturewhich adheres to a variety of surfaces with light pressure (fingerpressure) with no phase change (liquid to solid).

Acrylic Acid and Meth(acrylic) Acid Esters: The acrylic esters arepresent at ranges of from about 65 to about 99 parts by weight,preferably about 78 to about 98 parts by weight, and more preferablyabout 90 to about 98 parts by weight. Useful acrylic esters include atleast one monomer selected from the group consisting of a firstmonofunctional acrylate or methacrylate ester of a non-tertiary alkylalcohol, the alkyl group of which comprises from 4 to about 12 carbonatoms, and mixtures thereof. Such acrylates or methacrylate estersgenerally have, as homopolymers, glass transition temperatures belowabout −25° C. A higher amount of this monomer relative to the othercomonomers affords the PSA higher tack at low temperatures.

Preferred acrylate or methacrylate ester monomers include, but are notlimited to, those selected from the group consisting of n-butyl acrylate(BA), n-butyl methacrylate, isobutyl acrylate, 2-methyl butyl acrylate,2-ethylhexyl acrylate, n-octyl acrylate, isooctyl acrylate (IOA),isooctyl methacrylate, isononyl acrylate, isodecyl acrylate, andmixtures thereof.

Particularly preferred acrylates include those selected from the groupconsisting of isooctyl acrylate, n-butyl acrylate, 2-methyl butylacrylate, 2-ethylhexyl acrylate, and mixtures thereof.

Polar Monomers: Low levels of (typically about 1 to about 10 parts byweight) of a polar monomer such as a carboxylic acid can be used toincrease the cohesive strength of the pressure-sensitive adhesive. Athigher levels, these polar monomers tend to diminish tack, increaseglass transition temperature and decrease low temperature performance.

Useful copolymerizable acidic monomers include, but are not limited to,those selected from the group consisting of ethylenically unsaturatedcarboxylic acids, ethylenically unsaturated sulfonic acids, andethylenically unsaturated phosphonic acids. Examples of such monomersinclude those selected from the group consisting of acrylic acid (AA),methacrylic acid, itaconic acid, fumaric acid, crotonic acid, citraconicacid, maleic acid, .beta.-carboxyethyl acrylate, sulfoethylmethacrylate, and the like, and mixtures thereof.

Other useful copolymerizable monomers include, but are not limited to,(meth)acrylamides, N,N-dialkyl substituted (meth)acrylamides, N-vinyllactams, and N,N-dialkylaminoalkyl(meth)acrylates. Illustrative examplesinclude, but are not limited to, those selected from the groupconsisting of N,N-dimethyl acrylamide, N,N-dimethyl methacrylamide,N,N-diethyl acrylamide, N,N-diethyl methacrylamide,N,N-dimethylaminoethyl methacrylate, N,N-dimethylaminopropylmethacrylate, N,N-dimethylaminoethyl acrylate, N,N-dimethylaminopropylacrylate, N-vinyl pyrrolidone, N-vinyl caprolactam, and the like, andmixtures thereof.

Non-polar Ethylenically Unsaturated Monomers: The non-polarethylenically unsaturated monomer is a monomer whose homopolymer has asolubility parameter as measured by the Fedors method (see PolymerHandbook, Bandrup and Immergut) of not greater than 10.50 and a Tggreater than 15° C. The non-polar nature of this monomer tends toimprove the low energy surface adhesion of the adhesive. These non-polarethylenically unsaturated monomers are selected from the groupconsisting of alkyl(meth)acrylates, N-alkyl(meth)acrylamides, andcombinations thereof. Illustrative examples include, but are not limitedto, 3,3,5-trimethylcyclohexyl acrylate, 3,3,5-trimethylcyclohexylmethacrylate, cyclohexyl acrylate, cyclohexyl methacrylate, isobornylacrylate, isobornyl methacrylate, N-octyl acrylamide, N-octylmethacrylamide or combinations thereof. Optionally, from 0 to 25 partsby weight of a non-polar ethylenically unsaturated monomer may be added.

Tackifiers: Preferred tackifiers include terpene phenolics, rosins,rosin esters, esters of hydrogenated rosins, synthetic hydrocarbonresins and combinations thereof. These provide good bondingcharacteristics on low energy surfaces. Hydrogenated rosin esters andhydrogenated C9 aromatic resins are the most preferred tackifiersbecause of performance advantages that include high levels of “tack”,outdoor durability, oxidation resistance, and limited interference inpost crosslinking of acrylic PSAs.

Tackifiers may be added at a level of about 1 to about 65 parts per 100parts of the monofunctional acrylate or methacrylate ester of anon-tertiary alkyl alcohol, the polar monomer, and the nonpolarethylenically unsaturated monomer to achieve desired “tack”. Preferably,the tackifier has a softening point of about 65 to about 100.degree. C.However, the addition of tackifiers can reduce shear or cohesivestrength and raise the Tg of the acrylic PSA, which is undesirable forcold temperature performance.

Crosslinkers: In order to increase the shear or cohesive strength ofacrylic pressure-sensitive adhesives, a crosslinking additive is usuallyincorporated into the PSA. Two main types of crosslinking additives arecommonly used. The first crosslinking additive is a thermal crosslinkingadditive such as a multifunctional aziridine. One example is1,1′-(1,3-phenylene dicarbonyl)-bis-(2-methylaziridine) (CAS No.7652-64-4), referred to herein as “bisamide”. Such chemical crosslinkerscan be added into solvent-based PSAs after polymerization and activatedby heat during oven drying of the coated adhesive.

In another embodiment, chemical crosslinkers that rely upon freeradicals to carry out the crosslinking reaction may be employed.Reagents such as, for example, peroxides serve as a source of freeradicals. When heated sufficiently, these precursors will generate freeradicals, which bring about a crosslinking reaction of the polymer. Acommon free radical generating reagent is benzoyl peroxide. Free radicalgenerators are required only in small quantities, but generally requirehigher temperatures to complete the crosslinking reaction than thoserequired for the bisamide reagent.

The second type of chemical crosslinker is a photosensitive crosslinkerthat is activated by high intensity ultraviolet (UV) light. Two commonphotosensitive crosslinkers used for hot melt acrylic PSAs arebenzophenone and 4-acryloxybenzophenone, which can be copolymerized intothe PSA polymer. Another photocrosslinker, which can be post-added tothe solution polymer and activated by UV light is a triazine; forexample 2,4-bis(trichloromethyl)-6-(4-methoxy-phenyl)-s-triazine. Thesecrosslinkers are activated by UV light generated from artificial sourcessuch as medium pressure mercury lamps or a UV blacklight.

Hydrolyzable, free-radically copolymerizable crosslinkers, such asmonoethylenically unsaturated mono-, di- and trialkoxy silane compoundsincluding, but not limited to, methacryloxypropyltrimethoxysilane(SILANE™ A-174 available from Union Carbide Chemicals and Plastics Co.),vinyldimethylethoxysilane, vinylmethyldiethoxysilane,vinyltriethoxysilane, vinyltrimethoxysilane, vinyltriphenoxysilane, andthe like are also useful crosslinking agents.

Crosslinker is typically present from 0 to about 1 part by weight basedon 100 parts by weight of acrylic acid or meth(acrylic) acid esters,polar monomers, and non-polar ethylenically unsaturated monomers.

Aside from thermal, moisture, or photosensitive crosslinkers,crosslinking may also be achieved using high-energy electromagneticradiation such as gamma or e-beam radiation. In this case, nocrosslinker may be required.

Other Additives: Because acrylic pressure-sensitive adhesives haveexcellent oxidative stability, additives such as antioxidant and UVlight absorbers are generally not needed. Small amounts of heatstabilizer can be utilized in hot melt acrylic PSAs to increase thermalstability during processing.

Plasticizers: Optionally, low levels of plasticizer (e.g., less thanabout 10 parts by weight) may be combined with tackifier to adjust theTg in order to optimize the peel and the low temperature performance ofthe adhesive. Plasticizers that may be added to the adhesive of theinvention may be selected from a wide variety of commercially availablematerials. In each case, the added plasticizer must be compatible withthe tackified acrylic PSA used in the formulation. Representativeplasticizers include polyoxyethylene aryl ether, dialkyl adipate,2-ethylhexyl diphenyl phosphate, t-butylphenyl diphenyl phosphate,di(2-ethylhexyl)adipate, toluenesulfonamide, dipropylene glycoldibenzoate, polyethylene glycol dibenzoate, polyoxypropylene aryl ether,dibutoxyethoxyethyl formal, and dibutoxyethoxyethyl adipate.

Various polymeric film substrates comprised of various thermosetting orthermoplastic polymers are suitable for use as the overlay and bodylayer. The body layer may be a single layer or multi-layer film.Illustrative examples of polymers that may be employed as the body layerfilm for flexible retroreflective articles include (1) fluorinatedpolymers such as poly(chlorotrifluoroethylene),poly(tetrafluoroethylene-co-hexafluoropropylene),poly(tetrafluoroethylene-co-perfluoro(alkyl)vinylether), poly(vinylidenefluoride-co-hexafluoropropylene); (2) ionomeric ethylene copolymerspoly(ethylene-co-methacrylic acid) with sodium or zinc ions such asSURLYN-8920 Brand and SURLYN-9910 Brand available from E.I. duPontNemours, Wilmington, Del.; (3) low density polyethylenes such as lowdensity polyethylene; linear low density polyethylene; and very lowdensity polyethylene; plasticized vinyl halide polymers such asplasticized poly(vinychloride); (4) polyethylene copolymers includingacid functional polymers such as poly(ethylene-co-acrylic acid) “EAA”,poly(ethylene-co-methacrylic acid) “EMA”, poly(ethylene-co-maleic acid),and poly(ethylene-co-fumaric acid); acrylic functional polymers such aspoly(ethylene-co-alkylacrylates) where the alkyl group is methyl, ethyl,propyl, butyl, et cetera, or CH3 (CH2)n- where n is 0 to 12, andpoly(ethylene-co-vinylacetate) “EVA”; and (5) (e.g.) aliphaticpolyurethanes. The body layer is preferably an olefinic polymericmaterial, typically comprising at least 50 wt-% of an alkylene having 2to 8 carbon atoms with ethylene and propylene being most commonlyemployed. Other body layers include for example poly(ethylenenaphthalate), polycarbonate, poly(meth)acrylate (e.g., polymethylmethacrylate or “PMMA”), polyolefins (e.g., polypropylene or “PP”),polyesters (e.g., polyethylene terephthalate or “PET”), polyamides,polyimides, phenolic resins, cellulose diacetate, cellulose triacetate,polystyrene, styrene-acrylonitrile copolymers, cyclic olefin copolymers,epoxies, and the like.

Exemplary liners for use in the retroreflective articles of the presentdisclosure include silicone coated materials such as papers andpolymeric films, including plastics. The liner base material may besingle or multiple layer. Specific examples include, polyester (forexample polyethylene terephthalate), polyethylene, polypropylene(including cast and biaxially oriented polypropylene), and papers(including clay coated paper, polyethylene coated paper or apolyethylene coated poly(ethylene terephthalate) film.

In some embodiments, such as in retroreflective article 100, cube cornerelements 112 are in the form of a tetrahedron or a pyramid. The dihedralangle between any two facets may vary depending on the propertiesdesired in an application. In some embodiments (including the one shownin FIGS. 1A and 1B), the dihedral angle between any two facets is 90degrees. In such embodiments, the facets are substantially perpendicularto one another (as in the corner of a room) and the optical element maybe referred to as a cube corner. Alternatively, the dihedral anglebetween adjacent facets can deviate from 90° as described, for example,in U.S. Pat. No. 4,775,219, the disclosure of which is incorporated inits entirety herein by reference. Alternatively, the optical elements inthe retroreflective article can be truncated cube corners. The opticalelements can be full cubes, truncated cubes, or preferred geometry (PG)cubes as described in, for example, U.S. Pat. No. 7,422,334, thedisclosure of which is incorporated in its entirety herein by reference.Each retroreflecting optical element includes a symmetry axis that makesequal angles with the facets. In some embodiments, the symmetry axis isperpendicular to a base or front surface. In some embodiments, thesymmetry axis is not perpendicular to the base or the front surface andthe apex or optical element is canted as described, for example, in U.S.Pat. No. 4,588,258, the disclosure of which is incorporated in itsentirety herein by reference. Retroreflective layer 110 of FIGS. 1A and1B is shown as including overlay layer 118 and no land layer or landportion. A land layer may be defined as continuous layer of materialcoextensive with the cube corner elements and composed of the samematerial. This construction may be desirable for flexible embodiments.Those of skill in the art will appreciate that retroreflective layer 110can include a land layer or land portion.

As is schematically shown in FIG. 2, one method of making at least someof the retroreflective articles of the present disclosure involvesplacing barrier layer material 134 onto a pressure sensitive adhesivematerial 132 and then laminating the resulting pressure sensitiveadhesive layer 130 to a retroreflective layer 110. The pressuresensitive adhesive layer 130 can be formed in a variety of waysincluding but not limited to the following exemplary methods. In oneexemplary embodiment, the material(s) forming the barrier layer areprinted onto the pressure sensitive adhesive. The method of printing canbe, a non-contact method such as, for example, printing using an inkjetprinter. The method of printing can be a contact printing method suchas, for example, flexographic printing. In another exemplary embodiment,the material(s) forming the barrier layer are printed onto a flatrelease surface using, for example, an inkjet or screen printing method,and are then subsequently transferred from the flat release surface ontothe pressure sensitive adhesive. In another exemplary embodiment, thematerial(s) forming the barrier layer are flood coated onto amicrostructured adhesive surface (e.g., a Comply liner manufactured by3M Company of St. Paul, Minn.). The barrier layer material issubsequently transferred from the microstructured liner to the pressuresensitive adhesive by, for example, lamination. The retroreflectivearticle may then, optionally, be adhesively bonded to a substrate (e.g.,an aluminum substrate) to form, for example, a license plate or signage.

FIGS. 3 and 5 show some alternative exemplary retroreflective articlesof the present disclosure. Specifically, FIGS. 3 and 5 showretroreflective articles including structured sealing layers. In someembodiments the sealing layer includes at least one of, for example, athermoplastic polymer, a cross-linkable material, and a radiationcurable material. In some embodiments the sealing layer comprises anadhesive, such as, for example, a heat activated adhesive, and/or apressure sensitive adhesive. These constructions are characterized byhaving an embossed, replicated, or a similarly formed sealing layerlaminated to the back of the retroreflective layer. The sealing layercan be a pressure sensitive adhesive, heat activated adhesive, or othermaterial that can be formed using replication, heat embossing, extrusionreplication, or the like.

FIG. 3 is a schematic drawing of one exemplary embodiment of aretroreflective article 300 that faces viewer 302. Retroreflectivearticle 300 includes a retroreflective layer 310 including multiple cubecorner elements 312 that collectively form a structured surface 314opposite a major surface 316. Retroreflective layer 310 also includes anoverlay layer 318. Retroreflective layer 310 is shown as a flexiblesubstrate without a land layer or land portion, but, as is describedabove, retroreflective layer 310 can include a land layer and/or opticalelements of any type. A structured adhesive layer 330 is adjacent toretroreflective layer 310. Structured adhesive layer 330 includes raisedareas (a region that is raised relative to a surrounding region) ofadhesive in a closed pattern, such as, for example, a hexagonal array.Structured adhesive layer includes structured adhesive liner 340 and hotmelt adhesive layer 350. Structured adhesive layer 330, when bonded toretroreflective layer 310, defines low refractive index layers 338 thatretain the retroreflective nature of structured surface 314. Morespecifically, the presence of low refractive index layers 338 permit theportions of structured surface 314 adjacent to low refractive indexlayers 338 to retroreflect incident light 150. As such, portions ofretroreflective article 300 that include cube corner elements 312adjacent to low refractive index layers 338 are optically active in thatthey retroreflect incident light. In contrast, portions ofretroreflective article 300 that have portions of structured adhesivelayer 330 adjacent to cube corner elements 312 are optically inactiveareas in that they do not substantially retroreflect incident light.Portions of structured adhesive layer 330 that are not in contact withstructured surface 314 adhere to the cube corner elements 312, therebyeffectively sealing the retroreflective areas to form optically activeareas or cells. Structured adhesive layer 330 also holds the entireretroreflective construction together, thereby eliminating the need fora separate sealing layer and sealing process. In the embodiment shown inFIG. 3, retroreflective article 300 is adhesively bonded to an aluminumsubstrate 360 to form a license plate.

The structured adhesive layer can be formed in several different ways.The structured adhesive layer can include, for example, multiple layersformed at the same time or can be built through repeated coating steps.One exemplary method starts with a flat film of adhesive, optionally ona carrier web. The adhesive is nipped between a flat roll and a rollwith the required relief pattern. With the addition of temperature andpressure, the relief pattern is transferred to the adhesive. A secondexemplary method requires a castable or extrudable adhesive material. Afilm of the adhesive is created by extruding the material onto a rollwith the required relief pattern. When the adhesive material is removedfrom the roll, it retains the relief pattern associated with the roll.The structured adhesive layer is then laminated to the retroreflectivelayer.

In an alternative embodiment, the structured adhesive layers caninclude, for example, a material that is not an adhesive but is coatedwith an adhesive on the tips of the structure.

An exemplary method of making such a retroreflective article 400 beginswith a flat non-adhesive film such as, for example, polyethylene. Thepolyethylene film is nipped between a flat roll and a roll with therequired relief pattern. With the addition of temperature and pressure,the relief pattern is transferred to the polyethylene film. An adhesiveis then transferred to the tips of the replicated film using, forexample, kiss coating or another suitable method. The adhesive coveredstructured liner is then laminated to the retroreflector.

Regardless of which manufacturing method described above is used, thestructured adhesive layer is then bonded to the retroreflective layer bynipping the two films together in a nip consisting of two flat rolls.With the addition of temperature and pressure, the films adhesivelybond, creating pockets of air that retain the retroreflection of thecube corner elements.

Optionally, the first region or unraised portions of the adhesive can bepatterned with a material that acts to reduce the creep of thestructured adhesive layer seal legs, as well as minimizing thedetrimental affect of touchdown by the bottom of the well on the tips ofthe cube corner elements during processing or use. FIG. 5 shows aretroreflective article 500 in which a barrier layer 580 is limited tothe bottom of the structured adhesive layer, but barrier layer 580 couldbe anywhere in the wells as long as it does not substantially reduce theadhesion of the seal legs to retroreflective layer 310.

The structured adhesive layers of FIGS. 3-5 can include, for example, athermoplastic polymer, a heat-activated adhesive, such as, for example,an acid/acrylate or anhydride/acrylate modified EVA's such as, forexample, Bynel 3101, such as described in, for example, U.S. Pat. No.7,611,251, the entirety of which is herein incorporated by reference.The structured adhesive layers of FIGS. 3-5 can include, for example, anacrylic PSA, or any other embossable material with adhesivecharacteristics that will adhere to the corner cube elements. Theinterface between the seal film layer and the (e.g. cube-corner)microstructured layer typically include an adhesion promoting surfacetreatment. Various adhesion promoting surface treatments are known andinclude for example, mechanical roughening, chemical treatment, (air orinert gas such as nitrogen) corona treatment (such as described inUS2006/0003178A1), plasma treatment, flame treatment, and actinicradiation.

The coefficient of retroreflection R_(A), sometimes referred to asretroreflectivity of retroreflective articles, of the present disclosurecan be modified depending on the properties desired in an application.In some embodiments, R_(A) meets the ASTM D4956-07e1 standards at 0degree and 90 degree orientation angles. In some embodiments, R_(A) isin a range from about 5 cd/(lux·m²) to about 1500 cd/(lux·m²) whenmeasured at 0.2 degree observation angle and +5 degree entrance angleaccording to ASTM E-810 test method or CIE 54.2; 2001 test method. Insome embodiments, such as in embodiments where the retroreflectivearticle is used in a traffic control sign, a delineator, or a barricade,R_(A) is at least about 330 cd/(lux·m²), or at least about 500cd/(lux·m²), or at least about 700 cd/(lux·m²) as measured according toASTM E-810 test method or CIE 54.2; 2001 test method at 0.2 degreeobservation angle and +5 degree entrance angle. In some embodiments,such as in motor vehicle related applications, R_(A) is at least about60 cd/(lux·m²), or at least about 80 cd/(lux·m²), or at least about 100cd/(lux·m²) as measured according to ASTM E-810 test method or CIE 54.2;2001 test method at 0.2 degree observation angle and +5 degree entranceangle.

Another way of measuring these unique optical features of the sheetingof the present application involves measuring the fractionalretroreflectance R_(T). Fractional retroreflectance (R_(T)) is anotheruseful parameter for characterizing retroreflection. R_(T), which isexplained in detail in ASTM E808-01, is the fraction of unidirectionalflux illuminating a retroreflector that is received at observationangles less than a designated maximum value, α_(max). Thus, R_(T)represents the portion of light being returned within a prescribedmaximum observation angle, α_(max). In a manner consistent with ASTME808-01, R_(T) can be calculated as follows:

${R_{T} = {\int_{\alpha = 0}^{\alpha_{\max}}{\int_{\gamma = {- \pi}}^{\pi}{\left( \frac{R_{a}}{\cos(\beta)} \right)(\alpha)\ {\mathbb{d}\gamma}\ {\mathbb{d}\alpha}}}}},$where α is the observation angle (expressed in radians), γ is thepresentation angle (also expressed in radians), β is the entrance angle,and R_(a) is the conventional coefficient of retroreflection expressedin units of candelas per lux per square meter. For purposes of thisapplication, R_(T) refers to the fractional retroreflectance expressedas a decimal, and % R_(T) refers to the fractional retroreflectanceexpressed as a percentage, i.e., % R_(T)=R_(T)×100%. In either case, thefractional retroreflectance is unitless. As a graphical aid inunderstanding the observation angularity of a retroreflective sheeting,fractional retroreflectance may be plotted as a function of maximumobservation angle, α_(max). Such a plot is referred to herein as anR_(T)-α_(max) curve, or a % R_(T)-α_(max) curve.

Another useful parameter for characterizing retroreflection is R_(T)Slope, which can be defined as the change in R_(T) for a small change orincrement in the maximum observation angle, Δα_(max). A relatedparameter, % R_(T) Slope, can be defined as the change in % R_(T) for asmall change in maximum observation angle, Δα_(max). Thus, R_(T) Slope(or % R_(T) Slope) represents the slope or rate of change of theR_(T)-α_(max) curve (or % R_(T)-α_(max) curve). For discrete data pointsthese quantities may be estimated by calculating the difference in R_(T)(or % R_(T)) for two different maximum observation angles α_(max), anddividing that difference by the increment in maximum observation angle,Δα_(max), expressed in radians. When Δα_(max) is expressed in radians,R_(T) Slope (or % R_(T) Slope) is the rate of change per radian.Alternatively and as used herein, when Δα_(max) is expressed in degrees,R_(T) Slope (or % R_(T) Slope) is the rate of change per degree inobservation angle.

The equation given above for R_(T) involves integrating the coefficientof retroreflection R_(A) and other factors over all presentation angles(γ=−π to +π) and over a range of observation angles (α=0 to α_(max)).When dealing with discrete data points this integration can be performedusing R_(A) measured at discrete observation angle α_(max) values (0.1degrees) separated by increments Δα_(max).

In at least some embodiments of the present disclosure, the structuredsurface exhibits a total light return that is not less than about 5%,not less than 8%, not less than 10%, not less than 12%, not less 15% forincident visible light at an entrance angle of −4 degrees. In at leastsome of the embodiments of the present disclosure, the structuredsurface of the retroreflective article exhibits a coefficient ofretroreflection RA that is not less than about 40 cd/(lux·m2), not lessthan 50 cd/(lux·m2), not less than 60 cd/(lux·m2), not less than 70cd/(lux·m2), and not less than 80 cd/(lux·m2) for an observation angleof 0.2 degrees and an entrance angle of −4 degrees.

With appropriate choice of barrier layer materials, size, and/orspacing, the retroreflective articles of the present disclosure have amore uniform appearance than can be attained with conventionalretroreflective articles including a sealing layer. Additionally, theretroreflective articles of the present disclosure do not require theinclusion or use of a sealing layer, reducing their cost.

The embodiments including seal structures of the type shown in, forexample, FIGS. 3 and 5, have some additional specific advantages. Forexample, the seal legs can be made small enough so that they do notadversely affect the aesthetics of artwork or design printed on thesurface of the retroreflective article or construction. Further, thesemethods of sealing do not substantially change the angular distributionof retroreflected light from the bare retroreflective layer. Thesemethods also allow seal legs of arbitrary shape and color to be created.Consequently, retroreflective articles with a white appearance can beformed as well as articled with an anti-moire effect and/or securityfeatures. Lastly, the manufacturing process is streamlined because thevapor coat step is removed from the process.

Further, the retroreflective articles of the present disclosure haveimproved performance when compared to beaded sheeting. Prismaticsheeting is known in general to retroreflect a higher percentage of theincident light back towards the source when compared to glass bead basedsheeting. (See, e.g., FIGS. 2 and 3 of “Driver-Focused Design ofRetroreflective Sheeting for Traffic Signs”, Kenneth Smith, 87th AnnualMeeting of Transportation Research Board, Jan. 13-17, 2008, Washington,D.C.). When the retroreflected light is properly positioned with respectto observation angle, the result is a product with superior brightnessand performance.

Microsealed cell geometries, shapes, sizes and structures may be uniformacross the sheeting or may be varied in a controlled manner. Variationsin cell geometry such as size, shape, and cell width can be used tointentionally introduce slight fluctuations in visual appearance atclose distances (below 20 feet and preferably below 5 feet). Suchfluctuations can have the effect of hiding occasionally defects whichmight be randomly distributed in the sheeting (contaminants, coatingflaws, etc) or alternatively which might result from periodic structuresin the tooling or product (for example, scale up or weld lines).

Microsealed prismatic sheeting is especially suitable in applicationssuch as license plates and graphics. The prismatic sheeting providesbenefits such as significantly lower manufacturing cost, reduced cycletime, and elimination of wastes including especially solvents and CO₂when replacing glass bead sheeting. Furthermore, prismatic constructionsreturn significantly increased light when compared to glass beadretroreflectors. Proper design also allows this light to bepreferentially placed at the observation angles of particular importanceto license plates, e.g. the range 1.0 to 4.0 degrees. Finally, microsealed sheeting provides the brilliant whiteness and uniform appearanceat close viewing distances needed in these product applications.

Seal cells can be characterized by a cell size dimension and a seal cellleg or line width. The cell sizes for the 3M HIP and 3M flexibleprismatic products are about 0.143 inch and 0.195 inch, respectively.Seal leg widths for the same samples are about 0.018 inch and 0.025inch, respectively. Another way to characterize characteristic cell sizeis to divide cell area by perimeter length. This characteristic cellsize D_(c) may be useful in comparing different cell geometries. Theserelatively large seal cell sizes are perfectly suitable for theapplications in which they are normally utilized. Although thenon-uniform appearance of the retroreflective sheeting having these cellsizes is obvious when viewed up close (e.g., at a distance of a few feetor in a hand held or in-office demo). However, the actual distanceswhere this retroreflective sheeting is used are much greater. Forexample, critical sign distances (based on factors such as first andlast look distances, sign placement and obstruction and legibility) areabout 50 to 150 meters for right shoulder mounted signs. These criticaldistances represent the region where drivers are actually acquiring theinformation from highway signs. Individual seal cells are generally notvisible at these critical distances based on visual acuity thresholds.

At a distance of one foot, the normal visual acuity of the human eye isabout 0.0035 inch (88.9 micron). This means that if one had alternatingblack and white lines that were all about 89 micron wide, it wouldappear to most people as a mass of solid gray. (See,e.g.,http://www.ndted.org/EducationResources/CommunityCollege/PenetrantTest/Introduction/visualacuity.htm).Alternatively, 20/20 vision is the visual acuity needed to discriminatetwo points separated by 1 arc minute—about 1/16 of an inch at 20 feet(See, e.g.,http://en.wikipedia.org/wiki/Visual_acuity#Visual_acuity_expression).Hence for the approximate expected minimum viewing distances of abouttwo feet, any feature below about 180 microns can not be resolved.Sheeting with discrete feature sizes below this level would appearuniform to the human eye.

The alphanumeric letters used in license plates are relatively smallcompared to many road signing applications. For example, letter heightsof about 2.75 inch and stroke width (thickness of the alpha numerics)0.35 inch are common in the US. Similarly, letter heights of about 3.0inch and stroke width 0.45 inch are common in the US. Visual acuitylimits for black on white under ideal conditions, as described above,are about 0.0035 inch per foot of distance. Hence if the stroke width onUS plates is about 0.35 inch then the maximum distance where these canbe seen is about 100 feet. Contrast ratio between the alpha numeric andthe background may decrease due to factors such as dirt pick up orgraphic design (moving away from black letters on a white background).In such situations the maximum distance for reading the plate willfurther decrease. Real world variables such as poor illumination, movingvehicles, bad weather and poor eyesight can further and significantlyreduce maximum legibility distance. It is therefore common in thelicense plate market to study in particular the legibility of plates inthe critical license plate distance range of about 50 to 125 feet (15.2to 38.1 meters). The observation angles of particular importance tolicense plates are roughly in the range of about 1.0 to about 4.0degrees, as plotted in FIGS. 7A and 7B. In larger vehicles such as SUV'sor large trucks, the drivers eyes are even further from the headlightwhen compared to standard sedans. Hence, in larger vehicles in the samescenarios the observation angles will be even larger.

Exemplary retroreflective articles include, for example, retroreflectivesheeting, retroreflective signage (including, for example, trafficcontrol signs, street signs, highway signs, roadway signs, and thelike), license plates, delineators, barricades, personal safetyproducts, graphic sheeting, safety vest, vehicle graphics, and displaysignage.

The following examples describe some exemplary constructions of variousembodiments of the retroreflective articles and methods of making theretroreflective articles described in the present disclosure. Thefollowing examples are intended to be illustrative, but are not intendedto limit the scope of the present disclosure.

EXAMPLES

Preparation of Retroreflective Layer:

The overlay film was made by casting ethylene acid acrylate (EAA)(commercially available under the trade designation “Primacor 3440” fromDow Company of Midland, Mich.) as a film at a thickness of 0.01 cm (4mil) onto a corona treated polyethylene terephthalate (PET) carrierapproximately 53 in (134.6 cm) wide and 0.05 mm (0.002 in) thick.Pellets of EAA were fed into a 1.9 cm (¾ in) single screw extruderavailable from C.W. Brabender Instruments Inc., South Hackensack, N.J.The extruder temperature profile was from 140° C. (284° F.) to 175° C.(347° F.) resulting in a melt temperature of about 175° C. (347° F.). Asthe molten resin exited the extruder, it passed through a horizontal die(commercially available under the trade designation “Ultraflex—40” fromExtrusion Dies Industries LLC, Chippewa Falls, Wis.) and was cast ontothe PET carrier described above. The PET carrier was traveling atapproximately 36 meters/min (120 ft/min). The resulting molten overlayfilm on the PET carrier was run between a rubber roll and a chilledsteel backup roll to solidify the molten resin into a layer. The EAAsurface was corona treated at an energy of 1.5 J/cm².

The cube corner structure had three sets of intersecting grooves with apitch or primary groove spacing of 81.3 microns (0.0032 inch). Theintersecting grooves form a cube corner base triangle with includedangles of 61, 61, 58 degrees resulting in the height of the cube cornerelements being 37.6 microns (0.00148 inch). The primary groove spacingis defined as the groove spacing between the grooves which form the two61 degree base angles of the base triangle.

The cube corner microstructures were prepared using a resin compositionformed by combining 25 wt-% bisphenol A epoxy diacrylate (commerciallyavailable under the trade designation “Ebecryl 3720” from Cytek,Woodland Park, N.J.), 12 wt-% dimethylaminoethyl acrylate (“DMAEA”), 38wt-% TMPTA (trimethylol propane triacrylate) and 25 wt-% 1,6 HDDA(hexanediol diacrylate). The formulation had 0.5 pph of TPO(2,4,6-trimethylbenzoyl diphenylphosphine oxide) photoinitiator.

The resin composition was cast at room temperature at 25 fpm (7.6 m/min)onto a metal tool heated to 77° C. (170° F.). The resin compositionfilled the cavities of the cube corner microstructures in the tool via arubber nip roller having a gap set to fill the cavities of the embossedpattern on the tool and minimize the amount of resin on the land area ofthe tool. A retroreflective layer was made by contacting the coronatreated EAA film/PET carrier with the cube corner microstructures of theresin. The cube corner microstructure resin was cured through the PETcarrier/EAA film on the tool with twelve Fusion D UV lamp (availablefrom Fusion Systems, Rockville, Md.) set at 600 W/in. Dichroic filterswere used in front of the UV lamps to minimize IR heating of theconstruction. Upon completion of the curing process and removal of theretroreflective layer from the tool, the cube corner microstructureswere irradiated by a Fusion D UV lamp operating at 50% to provide apost-UV irradiation cure. The retroreflective layer was passed throughan oven set at 127° C. (260° F.) to relax the stresses in the film.

Examples 1-15

A structured layer was prepared by creating structures onto a substratelayer. In some embodiments, structures were created by selectivelyapplying (e.g., pattern printing, pattern coating) barrier materials(materials for use in the barrier layer) onto the substrate layer.Alternatively, barrier materials were applied on a release layerfollowed by lamination of the release layer containing the barriermaterials to the substrate layer. In some embodiments, the structuredlayer was prepared by imparting a pattern onto the substrate layer witha tool. In some embodiments, the substrate layer is an adhesive layer. Aretroreflective optical construction was prepared by laminating thestructured layer to the retroreflective layer, wherein the barriermaterials contacted the cube corner microstructures.

Comparative Examples A1 and A2

Retroreflective layers were prepared as described above, except thatdifferent lots of materials were used.

Examples 1-5

A radiation-polymerizable pressure sensitive adhesive (PSA) was preparedas described in U.S. Pat. No. 5,804,610 (Hamer), incorporated herein byreference. The PSA composition was made by mixing 95 parts by weightisooctyl acrylate (IOA), 5 parts by weight acrylic acid (AA), 0.15 partsby weight Irgacure 651 (commercially available from Ciba Corporation,now a BASF Company, NJ), 0.10 parts by weight4-acryloyl-oxy-benzophenone (ABP), 0.05 parts by weightisooctylthioglycolate (IOTG), and 0.4 parts by weight Irganox 1076(commercially available from Ciba Corporation). The PSA composition wasplaced into packages made of a ethylene vinyl acetate copolymer film of0.0635 mm thickness (commercially available under the trade designation“VA-24” from Pliant Corporation, Dallas, Tex.) measuring approximately10 centimeters by 5 centimeters and heat sealed. The PSA composition wasthen polymerized. After polymerization, the PSA composition wascompounded with 10% TiO₂ pigment and cast as a film onto a siliconerelease liner at a thickness of about 27 grains per 4 in by 6 in sample(11.3 mg/cm²), as generally described in U.S. Pat. No. 5,804,610. ThePSA film was then subjected to a radiation crosslinking step.

Barrier materials were selectively printed onto the PSA film using a UVinkjet printer (commercially available under the trade designation“Mimaki JF-1631” from Mimaki, Suwanee, Ga.). Yellow inkjet ink(commercially available from Mimaki) was used as the barrier material.The printer was run using 8 passes, unidirectional printing, with thelamps set on High, and with a resolution of 600×600 dpi. The ink levelwas set to 100% or 300% ink laydown. A printing pattern comprising dotsdisposed in a rhomboid parallelogram shape, wherein each dot wascentered in the vertex of the parallelogram, as schematically shown inFIG. 6, was used. Radius of the dots ranged from 400 to 600 μm, thedistance between the centers of horizontally adjacent dots was “S”(pitch), and the distance between the centers of vertically adjacentdots was “S”/2. “S” ranged from 1418 to 2127 μm. Width of the printingpattern was calculated by subtracting 2R from the pitch value. Coveragearea (% area) was calculated based on the relative amounts of printedand unprinted areas. Details on the patterns used to make PrintedAdhesive Layers 1-5 are shown in Table 1, below.

TABLE 1 Ink level, Pitch “S”, Dot Radius, Width, Pitch/width andCoverage Area for Printed Adhesive layers 1-5. Ink S level (pitch)Radius Width Pitch/ (%) (μm) (μm) (mm) Width % Area Printed Adhesive 1001418 400 618 2.29 50 Layer 1 Printed Adhesive 100 1554 500 554 2.81 65Layer 2 Printed Adhesive 100 1772 500 772 2.30 50 Layer 3 PrintedAdhesive 100 2127 600 927 2.29 50 Layer 4 Printed Adhesive 300 1772 500772 2.30 50 Layer 4

Retroreflective optical constructions (Examples 1-5) were prepared bylaminating Printed Adhesive Layers 1-5 to retroreflective layers using ahand squeeze roll laminator with a pressure setting of 40 psi (276 kPa),wherein the barrier materials contacted the cube corner microstructuresof the retroreflective layer.

Retroreflectivity (R_(A)) of the samples prepared as described inExamples 1-5 was measured at observation angles of 0.2, 1.0 and 2.0degrees, entrance angle of −4 degrees, and orientation of 0 degrees.Retroreflectivity of retroreflective layers (i.e., prior to laminatingprinted adhesive layers to the cube corner microstructures) (“Initial”),and Retroreflectivity of retroreflective optical constructions (i.e.,after lamination of printed adhesive) (“Laminated”) were measured, andare shown in Table 2, below.

TABLE 2 Retroreflectivity (R_(A)) of retroreflective layers (Initial)and retroreflective optical constructions (Laminated) prepared inExamples 1-5. Initial Laminated (cd/lux · m ²) (cd/lux · m²) Observationangle/Entrance angle (°) 0.2/−4 1/−4 2/−4 0.2/−4 1/−4 2/−4 Example 1 385130 14 73 26 4.2 Example 2 350 122 14 102 38 6.3 Example 3 373 124 13 6828 4.7 Example 4 347 123 12 70 27 4.5 Example 5 375 124 12 220 79 11

Formation of optically active areas was dependent on the number and/orsize of barrier materials printed on the PSA film. High coverage area(e.g., large number and/or large areas of barrier materials printed ontothe PSA film), result in the creation of a higher percentage ofoptically active areas, thus increasing retroreflectivity.

Examples 6

A PSA composition was prepared as described in Examples 1-5, except thatthe monomer ratio was 90/10 IOA/AA and no TiO₂ was used. The PSAcomposition was cast as a film at a thickness of 0.8 grains/in² (7.95mg/cm²).

Printed Adhesive Layer 6 was prepared by printing a barrier materialonto the PSA films using a square grid pattern, wherein each square was500 by 500 μm. Pitch (distance between the center of each adjacentsquare) was 700 μm. The distance between each square (width) was 200 μm.Pitch to width ratio was 3.5. Ink level was 300%. Area coverage (% area)was calculated based on the size of the sample and the printed pattern,and corresponded to 51%.

A retroreflective optical construction (Example 6) was prepared bylaminating Printed Adhesive Layer 6 to a retroreflective layer, asdescribed in Examples 1-5.

Retroreflectivity was measured at an observation angle of 0.2 degrees,entrance angle of −4 degrees, and orientation of 0 and 90 degrees, andis shown in Table 3, below.

TABLE 3 Retroreflectivity at 0 and 90 degrees orientation forComparative Example A, and Example 6. Retroreflectivity (cd/lux · m²)Orientation (degrees) 0 90 Comp. Ex. A1 124 124 Example 6 62 64

Example 7

Barrier materials were printed onto a silicone coated release layerusing the UV inkjet printer and yellow inkjet ink of Examples 1-5. Theink level used was 100%. The dot pattern described in Examples 1-5 wasused, and is schematically shown in FIG. 6. Printed Release Layer 7 wasprepared using the printing pattern detailed in Table 4, below.

TABLE 4 Distance “S”, Dot Radius and Coverage Area for Printed AdhesiveLayer 7. Radius S (μm) (μm) % Area Printed Release Layer 7 1418 400 50

Printed Adhesive Layer 7 was prepared by laminating Printed ReleaseLayer 7 to a PSA film prepared as described in Examples 1-5.Retroreflective Optical Construction (Example 7) was prepared bylaminating Printed Adhesive Layer 7 to a retroreflective layer.

Retroreflectivity was measured at observation angles of 0.2, 1 and 2degrees, entrance angle of −4 degrees, and orientation of 0 degrees, andis shown in Table 5, below.

TABLE 5 Retroreflectivity of Comparative Example A and Example 7.Initial Laminated (cd/lux · m²) (cd/lux · m²) Obs. angle/Ent. angle (°)0.2/−4 1/−4 2/−4 0.2/−4 1/−4 2/−4 Comparative Example 355 122 13 — — —A2 Example 7 360 130 13 59 21 3.5

Example 8

Barrier materials were created by screen printing (using an Accu-PrintPrinter available from A.W.T. World Trade, Inc., Chicago, Ill.) a whiteUV crosslinkable screen printing ink (commercially available under thetrade designation “9808 Series” from 3M Company, St. Paul, Minn.) onto asilicone coated release layer. Using a 380 mesh screen, the dot printingpattern of Examples 1-5 was used to create the barrier materials.Printed Release Layer 8 was prepared using the printing pattern ofExamples 1-5.

After screen printing, the barrier materials were cured using a UVcuring station made by American Ultraviolet Company, Murray Hill, N.J.,set at 226 mJ/cm² and run at 50 fpm (15.24 m/min).

Printed Adhesive Layer 8 was prepared by laminating Printed ReleaseLayer 8 to a PSA film. Retroreflective Optical Construction 8 wasprepared by laminating Printed Adhesive Layer 8 to a retroreflectivelayer.

Example 9

A structured layer was prepared as generally described in U.S. Pat. No.6,254,675 (Mikami), and incorporated herein by reference, using anembossing roll. The embossing roll was laser machined to provide apattern having the shape of a truncated, quadrangle pyramid having anexposed surface and a second quadrangle pyramid positioned on theexposed surface of the first pyramid, as shown in FIG. 4a of U.S. Pat.No. 6,254,675. The structure had a 200 μm pitch, 15 μm height, and 25 μmwidth, as generally described in Example 6 of U.S. Pat. No. 6,254,675. Apolyethylene coated paper release liner having a silicone coating overthe polyethylene, such as those available from Rexam or Inncoat, wasembossed between a heated rubber roll and the embossing roll to producea microstructured liner with ridges. The rubber roll was heated to atemperature of 110° C. and the release liner was heated to a surfacetemperature of 110° C. before entering the nip between the rubber rolland the embossing roll. The release liner traveled around approximatelyhalf of the embossing roll, and then onto a cold can which cooled theliner.

A coating composition comprised a vinyl solution with 10% solids,wherein a vinyl resin (commercially available under the tradedesignation “UCAR VYHH” from Dow Company, Midland, Mich.) was dissolvedin methyl ethyl ketone (MEK). The coating composition was coated ontothe structured layer to form Coated Release Layer 9. The line speed was5 fpm (1.52 m/min), and the pump flow rate was 5 ml/min and the coatingwas dried in an oven set at 170° F. (77° C.).

Printed Adhesive Layer 9 were prepared by laminating Coated ReleaseLayer 9 to a PSA film, prepared as described in Example 6. Aretroreflective optical construction (Example 9) was prepared bylaminating Printed Adhesive Layer 9 to a retroreflective layer.

Retroreflectivity was measured at observation angles of 0.2, 1 and 2degrees, entrance angle of −4 degrees, and orientation of 0 and 90degrees. Retroreflectivity is reported in Table 6 below, as an averageof retroreflectivity at 0 and 90 degrees orientation.

TABLE 6 Average Retroreflectivity of Examples 9. Retroreflectivity(cd/lux · m²) Obs. angle/ Ent. angle (°) 0.2/−4 1/−4 2/−4 Example 9 4223 11

Examples 10-13

The following description was used in preparing Examples 10-13: A laserablation system generally described in U.S. Pat. No. 6,285,001(Fleming), incorporated herein by reference, was used to impart apre-determined pattern to a polymeric film. The laser ablation systemcomprised a KrF excimer laser emitting a beam with a wavelength of lightof 248 nm, a patterned mask manufactured using standard semiconductorlithography mask techniques, and imaging lenses which projected an imageof the pattern of light passing through the mask onto a substrate. Thesubstrate comprised a 5 mil thick polyimide layer attached to a copperlayer. The substrate was exposed to patterned radiation and theresulting structure in the polyimide layer comprised a pattern ofhexagonal channels as shown in FIG. 4. Each hexagon was 0.731 mm wideand 0.832 mm long. The width of the channels, i.e., the distance betweeneach adjacent wall of a neighboring hexagon was 0.212 mm (width d), andthe distance between the center of one hexagon and the center of aneighboring hexagon was 0.943 mm (pitch D), as shown in FIG. 4A.

After cutting the pattern, a release treatment was applied to thesubstrate by first depositing a silicon containing film by plasmadeposition, followed by coating a fluorochemical composition. Plasmadeposition was carried out in a commercial Reactive Ion Etcher (model3032, available from Plasmatherm) configured with a 26 in lower poweredelectrode and central gas pumping. The substrate was placed on thepowered electrode and treated with an oxygen plasma by flowing oxygengas at a flow rate of 500 standard cm³/min and plasma power of 1000watts for 30 seconds. After the oxygen plasma treatment, a siliconcontaining diamond-like glass film was deposited by flowingtetramethylsilane gas at a flow rate of 150 standard cm³/min and oxygengas at a flow rate of 500 standard cm³/min for 10 seconds. Afterdeposition of the diamond-like glass film, the substrate was exposed toan oxygen plasma at a flow rate of 500 standard cm³/min for 60 seconds.A fluorochemical composition (commercially available under the tradedesignation “EGC-1720” from 3M Company, St. Paul, Minn.) was thenapplied to the substrate by manually dipping the substrate in thesolution and allowing it to dry. The substrate was then heated in anoven at 120° C. for 15 minutes.

Comparative Examples B1 and B2

Retroreflective layers were prepared as described in ComparativeExamples A1 and A2.

Example 10

A sealing layer comprising a two-layer construction of anacid/acrylate-modified ethylene vinyl acetate (EVA) polymer(commercially available under the trade designation “Bynel 3101” fromDow Corning, Mich., USA) was prepared by coextrusion, wherein a firstlayer was clear and a second layer was pigmented due to the addition ofTiO₂ during extrusion. Pellets of Bynel 3101 mixed with 20% by weight ofa 80/20 TiO₂/EVA blend were fed into the extruder and cast as a whitefilm at a thickness of 0.005 cm (2 mil) onto a PET carrier. The clearlayer (i.e., without TiO₂) was cast as a film at a thickness of 0.002 cm(1 mil) and corona treated at an energy of about 1 J/cm².

Structures were created on the sealing layer by pressing the clear Bynelside of the previously described multilayer film onto the patternedpolyimide layer in a hot press (commercially available under the tradedesignation (model “PW-220H” available from IHI Corporation, Houston,Tex.) for 3 minutes. Embossing temperatures for the top and bottomportions of the press were of about 230° F. (110° C.), and embossingpressure was 10 psi (69 kPa).

The structured sealing layer was subsequently laminated to theretroreflective layer, with the clear film adjacent the cube cornermicrostructures, forming a retroreflective optical construction. A heatpress (model “N-800” from HIX Corporation, Pittsburg, Kans.) was usedwith a lamination temperature about 200° F., pressure of 30 psi (207kPa), for 30 seconds.

Example 11

A sealing layer was prepared as described in Example 10, except that thestructured sealing layer was coated with a low index coating materialprior to lamination to the retroreflective layer.

The low index coating composition was prepared using a non-surfacemodified alkaline stabilized dispersion of M-5 silica (commerciallyavailable under the trade designation “Cabo-Sperse PG002” from Cabot ofBellerica, Mass.) and a polyvinyl alcohol (PVA) (commercially availableunder the trade designation “Poval 235” from Kuraray USA). This silicais characterized by its low surface area which is typically about 80-120m²/g. To a 1000 ml plastic beaker were added 150 g of a 7.2% solids PVAsolution in water, 2.0 g of a nonionic surfactant (commerciallyavailable under the trade designation “Tergitol Min-Foam 1X” from DowChemical Company, Midland, Mich.), and 1 ml of a concentrated NH₄OHsolution. The solution was mixed at low shear using an air poweredoverhead laboratory mixer operating at low speed. The silica dispersion(216 g, 20% weight percent in water) was added to the solution, followedby the addition of 130 g of deionized water. The blend was allowed tomix for approximately 15 minutes. The blend, comprising 1 part of PVA to4 parts silica on a dry weight basis, was then transferred to a 1 Lround bottom flask and placed on a rotary evaporator at a temperature ofabout 40° C. and 600 mmHg vacuum. The final solids content of the lowindex coating composition was adjusted to 5% using deionized water.

The low index coating composition was coated onto the structured side ofthe sealing layer using a knife coater. The knife was set to providezero land in the coating, meaning that excess silica was removed fromthe sealing layer. The sealing layer was placed in an oven at 50° C. for10 minutes prior to lamination to the cube corner microstructures.

Retroreflectivity was measured at an observation angle of 0.2 degrees,entrance angle of −4 degrees, and orientation of 0 and 90 degrees.Retroreflectivity is reported in Table 7 below, as an average ofretroreflectivity at 0 and 90 degrees orientation.

TABLE 7 Average Retroreflectivity of Comparative Example B1, andExamples 10-11. Retroreflectivity (cd/lux · m²) Observationangle/Entrance angle (°) 0.2/−4 Comparative Example B1 434 Example 10388 Example 11 381

Example 12

A PSA film was prepared as described in Examples 6. A structuredadhesive film was obtained by pressing the adhesive film against thepolyimide layer, as described in Example 10.

A retroreflective optical construction was prepared by laminating thestructured adhesive film to the retroreflective layer, wherein thestructured adhesive contacted the cube corner microstructures.Lamination occurred at room temperature using a hand roller.

Example 13

A structured adhesive film was prepared as described in Example 12,except that the low index coating composition of Example 11 was coatedonto the structured adhesive. A retroreflective optical construction wasprepared by laminating the coated structured adhesive film to theretroreflective layer at room temperature with a hand roller, whereinthe adhesive side of the film contacted the cube corner microstructures.

Retroreflectivity was measured at an observation angle of 0.2 degrees,entrance angle of −4 degrees, and orientation of 0 and 90 degrees.Retroreflectivity is reported in Table 8 below, as an average ofretroreflectivity at 0 and 90 degrees orientation.

TABLE 8 Average Retroreflectivity of Comparative Example B2, Examples12-13. Retroreflectivity (cd/lux · m²) Observation angle/Entrance angle(°) 0.2/−4 Comparative Example B2 212 Example 12 120 Example 13  64

Comparative Example C

A sealed prismatic retroreflective sheeting commercially available underthe trade designation “Diamond Grade 3910” by 3M Company, St. Paul,Minn., was obtained.

Comparative Example D

A beaded retroreflective sheeting commercially available under the tradedesignation “License Plate Sheeting Series 4770” by 3M Company, wasobtained.

Comparative Example E

A sealing layer prepared as described in Example 10 was obtained.

Comparative Examples F, G and H, and Examples 14-19

The following description was used in preparing Comparative Examples F,G, and H, and Examples 14-19: A retroreflective layer was prepared asdescribed in “Preparation of Retroreflective Layer”, except that thecube structure had three sets of intersecting grooves with a pitch orprimary groove spacing of 101.6 microns (0.004 inch). The intersectinggrooves form a cube corner base triangle with included angles of 58, 58,64 degrees resulting in the height of the cube corner elements being49.6 microns (0.00195 inch). The primary groove spacing is defined asthe groove spacing between the grooves which form the two 58 degree baseangles of the base triangle.

Comparative Example F and G were prepared as described in “Preparationof Retroreflective Layer”, except that the above-mentioned cube cornerstructure was used.

Comparative Example H was prepared by laminating the retroreflectivelayer of Comparative Examples F and G to the sealing layer of Example10.

Polyimide layers (Tools 1-6) were prepared, except that the patternshape was that of a diamond, shown in FIG. 8 and FIG. 9, wherein angle 1(A1) and angle 2 (A2) were respectively 20 and 153 degrees. Dimensionalcharacteristics of Tools 1-6 are shown in Table 9, below. The sealedretroreflective sheeting of Comparative Example C was analyzed and thepitch and line width of the seal pattern was measured, and is reportedin Table 9, below.

TABLE 9 Dimensional characteristics of Tools 1-6, and ComparativeExample C. Line width Area/ d/D (pitch/ % (μm) Pitch (μm) perimeterwidth) coverage Tool 1 260 680 205 2.61 31 Tool 2 200 680 205 3.40 37Tool 3 193 680 205 3.52 37 Tool 4 156 680 205 4.36 45 Tool 5 225 1165343 5.18 59 Tool 6 210 1265 316 6.02 64 Comp. 685 5511 1294 8.04 75.12Example C

First generation negative toolings were made from the polyimide layers(master) by nickel electroforming the polyimide layer in a nickelsulfamate bath as generally described in U.S. Pat. No. 4,478,769(Pricone) and U.S. Pat. No. 5,156,863 (Pricone). Additionalmultigenerational positive and negative copies were formed such that thetooling had substantially the same geometry as the master.

Structured Sealing layers 14-19 were prepared by pressing the sealinglayer of Example 10 on Tools 1-6, as generally described in Example 10.

Retroreflective optical constructions (Examples 14-19) were prepared bylaminating Structured Sealing layers 14-19 to retroreflective layers.

Retroreflectivity (R_(A)) was measured at observation angles of 0.2,0.5, 1, 2, 3, and 4 degrees, entrance angle of −4 degrees, andorientation of 0 and 90 degrees. Retroreflectivity is reported in Table10 below, as an average of retroreflectivity at 0 and 90 degreesorientation.

TABLE 10 Average Retroreflectivity of Comparative Examples C, D, F, andG, and Examples 14-19 RA (cd/lux · m2) Observation angle (°) 0.2 0.5 1 23 4 Example 14 167 69 50 10 4 4 Example 15 205 87 56 10 4 4 Example 16207 84 60 12 4 4 Example 17 305 128 74 13 5 4 Example 18 268 116 97 17 55 Example 19 324 140 103 17 5 5 Comparative Example C 799 586 47 13 3 4Comparative Example D 110 48 17 8 4 4 Comparative Example F 547 239 15920 6 5 Comparative Example G 648 258 140 20 7 5

Retroreflectivity (R_(A)) is typically measured at discrete observationangles and averaged over the annular region between two adjacentmeasured observation angles. Incremental % RT for a given observationangle is determined by multiplying this average R_(A) by the area ofthis annular region divided by the cosine of the entrance angle.Fractional retroreflectance % RT is the sum of incremental % RT forobservation angles between 0 and the observation angle of interest(amax). Fractional retroreflectance slope (% RT slope) for a givenobservation angle is the incremental % RT divided by the differencebetween the adjacent observation angles. % RT slope was calculated andis reported in Table 11 below.

TABLE 11 % RT slope of Comparative Examples C, D, F, and G, and Examples14-19. % RT Observation angle (°) 0.2 0.5 1 2 3 4 Example 14 4.7% 6.1%10.4% 4.0% 2.0% 1.7% Example 15 5.9% 7.4% 12.2% 4.2% 1.9% 1.6% Example16 5.9% 7.4% 12.6% 4.8% 2.4% 1.9% Example 17 8.9% 10.4% 16.7% 5.6% 2.3%1.8% Example 18 7.5% 10.6% 18.4% 6.7% 3.1% 2.1% Example 19 9.2% 12.4%20.9% 7.0% 3.0% 2.1% Comparative 23.4% 46.5% 13.8% 3.6% 1.70%  1.60% Example C Comparative 2.3% 4.3% 2.4% 2.1% 1.0% 0.5% Example DComparative 15.9% 20.6% 33.0% 9.2% 3.5% 2.2% Example F Comparative 19.1%21.3% 31.6% 9.4% 3.9% 2.6% Example G

% RT was calculated and is reported in Table 12 below.

TABLE 12 % RT of Comparative Examples C, D, F and G, and Examples 14-19.% RT Slope Observation angle (°) 0.2 0.5 1 2 3 4 Example 14 0.6% 2.5%6.8% 12.5% 15.3% 16.9% Example 15 0.7% 3.1% 8.3% 14.8% 17.5% 19.0%Example 16 0.7% 3.1% 8.4% 15.2% 18.5% 20.4% Example 17 1.1% 4.5% 12.0%20.5% 23.9% 25.8% Example 18 0.9% 4.2% 11.6% 21.8% 26.1% 28.5% Example19 1.1% 5.0% 13.7% 24.5% 28.8% 31.1% Comparative 3.0% 14.4% 29.3% 35.0%37.6% 39.0% Example C Comparative 0.3% 1.5% 2.9% 5.7% 7.2% 7.9% ExampleD Comparative 1.9% 8.3% 22.8% 38.6% 43.9% 46.4% Example F Comparative2.4% 9.4% 24.2% 38.6% 44.3% 47.2% Example G

CAP-Y was measured on a color measurement spectrophotometer (availableunder the trade designation “ColorFlex” from Hunterlab, Reston, Va.).All samples were measured with a white background behind the sample.There is a linear relationship between CAP-Y and % coverage (i.e., %area that is retroreflective), as increased % coverage results indecreased CAP-Y. Decreasing the amount of cube corner microstructuresthat are exposed (i.e., decreasing % coverage), results in an increasein whiteness of the construction. Typical CAP-Y values for license platesheeting are equal to 50 or greater. CAP-Y values of ComparativeExamples C, E, F, and H, and Examples 14-19, are shown in Table 13,below.

TABLE 13 CAP-Y values of Comparative Examples C, E, F, and H, andExamples 14-19 CAP-Y % coverage Example 14 70.93 31 Example 15 68.7 37Example 16 64.97 37 Example 17 62.63 45 Example 18 59.49 59 Example 1957.09 64 Comparative Example C 53.97 75 Comparative Example E 90.59 0Comparative Example F 41.15 100 Comparative Example H 85.94 0

As shown in Tables 10, 12, and 13, a decrease in the amount of exposedcube corner microstructures (e.g., by printing or laminating astructured film to the cube corner microstructures) results in adecreased amount of light returned, as seen in fractionalretroreflectance % RT at 2, 3 and 4 degrees observation angle.

Another way to characterize the dimension of the samples is to use theseal cell pitch to line width ratio (D/d). The pitch D is the distancebetween the smallest repeating features in the pattern. It can also becalled seal cell size or characteristic cell size. The width d is thewidth of the seal legs. The effect of the pitch/width ratio (D/d) onCAP-Y and % RT slope at 2, 3 and 4 degrees is shown in Tables 10, 11,and 13.

Microsealed retroreflective optical constructions are particularlyuseful for providing brilliant whiteness and uniform appearance at closeviewing distances while exhibiting excellent retroreflectiveperformance. One additional important aspect of the small or micro sealcell approach is that as the geometry of the seal cells (e.g., cell size(D), leg width (d)) and the total amount of light returned is reduced,the light distribution remains relatively unchanged. Sealedretroreflective optical constructions show a drop in total light return,however the light distribution profile remains the same.

As it will be understood by one of ordinary skill in the art, the shapeof the seal cell is not limited to the diamond shapes presented in theabove examples. The shape of seal cells may include, for example,circles, ovals, triangles, squares, and parallelograms. Virtually anyshape that meets the characteristic dimension, % coverage, orpitch/width considerations shown above, or in any combination thereof,could be used. Seal cells could also have the shape of letters orimages, such as an inverted ‘M’ or ‘3M’ as security features.

Any geometric parameter used to define the seal cell shape includingpitch, line width, angle, straightness, can be randomized to reduce theeffects of moiré between the seal cells and the retroreflectivesheeting. The geometry could also be optimized to hide defects in thesheeting, as well as layup or weld lines.

Additionally, the size or pitch, P, of the cubes may affect thebrightness when laminated to the sealing layer due to the formation ofedge effects during sealing. Two parameters can be used to helpunderstand cube size effects: actual cell line width (d) and aneffective cell line width (deff). The actual line width d is thephysical width of the material that creates the seal cell leg. Theeffective line width deff is the optical width of the seal legs, and isdependent on the size and geometry of the cube corners used in theprismatic film to which the sealing layer is laminated. When the sealinglayer is laminated to the retroreflective layer, the part of the sealleg that overlaps only part of the cube corner microstructure darkensthe whole cube. This additional darkened area makes the effective widthwider than the actual width. This effect is very dependent on the sizeof the cubes, the effect being much worse for large cubes.

The recitation of all numerical ranges by endpoint is meant to includeall numbers subsumed within the range (i.e., the range 1 to 10 includes,for example, 1, 1.5, 3.33, and 10).

All references mentioned herein are incorporated by reference.

Those having skill in the art will appreciate that many changes may bemade to the details of the above-described embodiments andimplementations without departing from the underlying principlesthereof. Further, various modifications and alterations of the presentinvention will become apparent to those skilled in the art withoutdeparting from the spirit and scope of the invention. The scope of thepresent application should, therefore, be determined only by thefollowing claims.

What is claimed is:
 1. A method of forming a retroreflective article,comprising: providing a retroreflective layer including a structuredsurface that is opposite a second major surface; providing a structuredadhesive layer having raised areas and unraised areas, wherein at leastsome of the unraised areas further include barrier layers; and attachingthe structured adhesive layer to the structured surface such that theraised areas are in contact with the structured surface and the unraisedareas are not in contact with the structured surface.
 2. The method ofclaim 1, wherein multiple raised areas surround unraised areas to form adiscrete first region.
 3. The method of claim 1, wherein the structuredadhesive layer is adjacent to a liner.